The present invention relates to an image scanner capable of scanning images formed on both sides of a sheet at the same time.
Recently, an image scanner capable of simultaneously scanning images formed on both sides of a sheet has become widespread. In such an image scanner, a CCD (Charge Coupled Device) type image sensor is provided along a sheet path inside the scanner to scan an image formed on one side of sheet, and a CIS (Contact Image Sensor) type image sensor is provided along the sheet path to scan an image formed on the opposite side of the sheet. The image scanner typically includes an ADF (Automatic Document Feeder), and therefore, in order to provide a sufficient room for the ADF and various other peripheral devices, the CIS type image sensor is used as one of the image sensors.
The CIS type image sensor (hereinafter occasionally referred to as a first sensor) scans, for example, a back surface of a sheet to capture an image formed thereon, and the CCD type image sensor (hereinafter occasionally referred to as a second sensor) scans, for example, a front surface of the sheet to capture an image formed thereon.
Each of the output ports of the first and second image sensors is connected with an image processing unit, which typically includes a gain adjusting circuit, A/D converter, compensation circuit and the like for receiving an image (component) signal output therefrom. The image signals (or image component signals) output from the output ports of the sensors are different in terms of a gain adjustment range and compensation parameters used in the compensation circuit. Therefore, the image processing units are provided for respective image signals (image component signals).
With the above-described configuration, when an image formed on one surface of a sheet is scanned to capture a color image, an image sensor provided with RGB (Red, Green and Blue) color filters is used, and the image processing units respectively connected to the output ports, which respectively output RGB color components, of the image sensor are used. When an image formed on one surface of a sheet is scanned to capture a monochromatic image, for example, the image sensor is used for scanning, however, only the image processing unit connected to the output port of, for example, the G (green) component of the color image sensor is used. If both surfaces of a sheet are simultaneously scanned, the image processing units connected to the output ports of both the first and second image sensors are necessary.
It is understood that the image sensor for scanning the back surface of a sheet is used only when both the surfaces of the sheet are scanned, and the image processing unit for this sensor is used only when both the surfaces of the sheet are scanned. Therefore, the conventional image scanner capable of scanning both surfaces of a sheet includes excessive parts, which increases a manufacturing cost.